Fill in ActivatableNotificationViewController.

This reverts commit 1a2744a4006c61b1f1f29e8c8def7f6ebf65e14a.

Prior commit had a bug related to clicking on notifications
that is now addressed.

Bug: 14724574
Test: atest SystemUITests
Change-Id: Ie1433a6e2c3095e31c9184032510b3db75de2ef3
7 files changed